Should signed text messages use the "\x19Ethereum Signed Message" prefix?

Question

Background: I want to implement sign message feature in Trezor.

Is there a standard for signing text messages?

So far I found out that etherscan.io signs messages without the prefix, i.e. it signs keccak(msg), but also accepts signatures "with Geth prefix" of keccak("\x19Ethereum Signed Message\n32"+keccak(msg)). Signed messages of the form keccak("\x19Ethereum Signed Message\n"+strlen(msg)+msg) are not accepted.

MyEtherWallet only accepts messages signed without any prefix.

Without the prefix one can misuse the sign message feature to sign a transaction, though. Is this intentional?

Answer 1

There's no official standard (at the time of this writing) and has been debated for a while in this EIP thread EIP #683

According to issue #3731:

Geth prepends the string \x19Ethereum Signed Message:\n<length of message> to all data before signing it (https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign). If you want to verify such a signature from Solidity, you'll have to prepend the same string in solidity before doing the ecrecovery.

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Answer 2

I think it's not save to always use the same prefix when signing. For example if you have deployed the smart-contract multiple times, signatures can be replayed on each instance. You should always use a custom prefix and also add the contracts address to the hashing before you sign something. For javascript you could use eth-crypto to sign signs with a custom prefix, like described in the tutorial.

Answer 3

Yes, you should use the "\x19Ethereum Signed Message" prefix.

Trezor One adds the prefix itself: https://github.com/trezor/trezor-firmware/blob/be76dcc96a09cdbb807e6a1df531912a5be20aee/legacy/firmware/ethereum.c#L631

Source

Answer 4

However Trezor adds the prefix auto, but wherever its required you to manually enter it for strlen(msg),it should be "32" (in string) or better to use "\x19Ethereum Signed Message:\n32" + keccak256(msg)